This change does replacement of ARIA roles that have newer synonyms.
There are a number of newer ARIA roles that are synonyms for older
roles. https://wpt.fyi/results/wai-aria/role/synonym-roles.html has a
number of subtests which expect that when retrieving the value of an
explicitly- specified role attribute, if the value is one of the older
role values, implementations must replace that with its newer synonym.
This change separates the steps for checking the string value of the
ARIA “role” attribute out from the element.role_or_default() function
into a separate function — in order to expose a way to just check if the
ARIA “role” attribute actually has a value, without also then computing
a default role value if no “role” attribute value was found.
Otherwise, without this change, the only available function for
retrieving ARIA role values is the element.role_or_default() function —
which always does the additional step of computing (and returning) a
default role value if no “role” attribute is found.
